The Petrofka bridge landslide : a case history study

An investigation of a multiple retrogressive landslide at the site of the Petrofka Bridge provided a unique opportunity to identify the failure mechanism and evaluate the present stability of the landslide. Although aggrading of the North Saskatchewan River following deglaciation has resulted in stabilization of certain landslide styles while altering the mechanism of failure in others, the present stability of this landslide was supported by over twenty years of satisfactory performance of the Petrofka Bridge. The landslide at the Petrofka Bridge is approximately 900 metres long, 60 metres high and up to 110 metres thick. The shear zone is in a montmorillonitic clay shale of the Tertiary-Quarternary sediments. The pore-water pressures at the shear zone were controlled by pore-water pressure conditions within the artesian sandstone of the Judith River Formation. The artesian pore-water pressures within the Judith River Formation were controlled by lithostatic pressures of the overlying sediments during deepening of the valley, as well as interception of the Judith River Formation by the river alluvium upstream and downstream of the study site. The landslide at this site was formed as a direct result of downcutting of the river valley during deglaciation combined with lithostatic pressures within the underlying sandstone of the Judith River Formation and residual strength properties of the sediments at the shear zone. Interception of the Judith River Formation by the river thalweg upstream and downstream of the study site resulted in stabilization of this landslide during the latter stages of deglaciation. Aggrading of the North Saskatchewan River with the deposition of up to 33 metres of alluvial sand followed by increasing artesian pore-water pressures at less than lithostatic pressure within the Judith River Formation subsequent to deglaciation has maintained the stability of this landslide. A laboratory investigation using a direct shear test indicated the residual strength of the clay shale at the shear zone included a residual angle of shearing resistance of 6.0 degrees with zero cohesion. A comprehensive back-analysis performed on the landslide at this site yielded values of residual strength parameters of internal angle of shearing resistance of 5.3° to 5.8° with zero cohesion for limiting equilibrium of the entire mass acting as one coherent unit. The present day stability of the landslide at this site is considered to be represented by a factor of safety of 1.10